1. Field of the Invention
The invention pertains to the field of quality control of fasteners and other fabricated components. The invention addresses the problem of providing rapid determination of component acceptability using a component imaging system.
2. Brief Description of Prior Art
In the fastener art, the quality of individual fasteners may be assessed through analysis of fastener characteristics such as shank diameter, perpendicularity, and eccentricity, as well as certain specific thread measurements such as minor and major diameter, thread pitch, pitch diameter, flank angle, length of thread engagement and the like. By comparing the actual characteristics of a fastener to maximum and minimum tolerances for each of its characteristics, the quality of the fastener may be determined.
However, the degree to which such quality control is practical depends on the time required to make these assessments. Conventional technology requires at least one minute to accurately gage a fastener's dimensions. Using conventional technology, it is necessary to make tradeoffs between quality assurance measurement and production speed, for example by taking fewer than all the measurements that would be advantageous from a quality assurance standpoint, or by sampling individual fasteners and generalizing the characteristics of that fastener to all fasteners from the same production batch.
One approach to determining fastener quality involves generating an image of a sample fastener and analyzing the image using a computer. The teachings of certain relevant prior art documents and other conventional knowledge in the field is now briefly reviewed.
U.S. Pat. No. 3,941,484--Dreyfus, discloses backlighting an object by use of a laser and a collimating lens or mirror that expands the point source laser light to encompass an object to be examined. The object partly occludes the expanded beam and provides an image. Additional lenses concentrate the beam at a point or scanning tube arrangement, such that the luminance of the collected light is serialized like a raster image. The serial luminance level is sampled repetitively in conjunction with a threshold comparator and timing means to locate the edge of the object from a change in the amplitude of the luminance signal. The comparator data, in connection with timing data, can be converted into a dimensional measurement. This scanning arrangement is inherently serial, and as a result, the scanner can sense light intensity from the beam at only one position at a time. Timing considerations affect the accuracy of the measurement. Moreover, the system is prone to error if applied to moving parts.
U.S. Pat. Nos. 4,576,482 and 5,114,230, both to Pryor, disclose systems in which a collimated laser light is used as a back-light to collect luminance information by finding the transition in luminance across a single edge of an object. The light is partly occluded by the object and is applied to a linear photodiode array having an extension perpendicular to the edge. The object can be a cam or the like that is rotated in conjunction with the measurement to define the contour of the cam surface. The Pryor technique is also serial in that it detects the luminance transition along a line. The collimated beam is not wide enough to profile both edges of the object simultaneously. Furthermore, an array is not used to detect the resulting silhouette.
U.S. Pat. No. 4,315,688--Pryor discloses utilizing reflected light rather than an occluded backlight source, to check the quality or at least presence of fastener threads. Reflected light typically produces low accuracy measurements when used to gage fastener dimensions. An image of a portion of the threads of a fastener is created on a single sensor and the output signal is analyzed to determine thread quality.
Thread quality gaging is a particularly demanding imaging problem that has not been solved completely by digital imaging methods, especially where use of a lens is involved. Conventional technology requires accurate positioning of the focusing lens a certain distance from the object being measured in order to produce a silhouette identical in size to the object.
U.S. Pat. 4,644,394--Reeves discloses a system for examining external threads on a pipe using a collimated laser beam to back-light the threads along one side of the pipe. A luminance transition thus is detectable at a tangent of the pipe surface. An image of the pipe threads at one angular point on one side of the pipe is collected and processed and enables certain thread measurements in that area. However, in order to make related thread measurements at other angular points around the pipe, for example to determine pipe diameter or to relate thread data in different areas, it is necessary to rotate the pipe or to move the camera around the pipe axis while keeping the collimated source aligned to the camera. A metered rotation drive means can be used to rotate the pipe or to orbit the camera and source, so as to enable the data applicable to different points along the threads to be correlated. However, system accuracy is highly dependent upon the rotation drive means and the ability to accurately position the part at different angular positions with respect to the rotation axis. These requirements introduce time constraints and measurement inaccuracies.
Three patents assigned to the Boeing Company (U.S. Pat. Nos. 5,150,623, 4,828,159 and 4,823,396) disclose digital imaging methods which utilize back lighting to gage fastener dimensions. U.S. Pat. No. 4,823,396--Thompson discloses a method in which a fastener is back-lit by an array of LED's (light emitting diodes). A video image is created of the fastener profile, which is then digitized and processed to verify thread presence, discriminate between fasteners with helical and parallel threads, measure thread length and perform various dimensional measurements on the head and shank of the fastener. The accuracy of the measurements performed is dependent on the distance between the camera lens and the fastener. Without collimated back lighting, slight variations in distance generate variations in image feature size and consequently measurement errors. Such uncertainties are significant when fasteners must be dimensioned to within tight tolerances, for example for use in the aerospace industry.
U.S. Pat. Nos. 4,828,159 and 5,150,623 both issued to Woods, disclose methods which attempt to remedy the above problem by using two cameras, imaging the fastener from two orthogonal directions and mathematically calculating the position of the fastener at the time of imaging. The calculation of precise fastener position is dependent upon precise focussing of the cameras. Minute focusing errors generate errors in the calculated position, and hence in the detected fastener dimensions.
It is known to collect and analyze a two dimensional image of an object by opto-electronic methods employing digital video imaging means, and in particular to perform dimensional measurements. In conventional systems, an image of an object or its profile is focused on a two dimensional detector array such as that of a video camera. The image is digitized, often together with conventional image enhancement techniques intended to produce better edge contrast. A computer or other electronic processing device processes the digital information to locate edges and other relevant image points by finding transitions in luminance. Various object dimensions are then determined from the pixel position displacements between transitions in luminance. With a proper setup, measurements can be made quickly and accurately, even of objects moving rapidly on an assembly-line conveyor. The accuracy of the measurements performed with such methods is limited by several factors, including the fidelity of the input image, the manner of illumination, the imaging optics, the detector, and the placement of the object to be measured.